WO2019069104A1

WO2019069104A1 - Apparatus and method for monitoring a temporary perimeter that delineates between an open carriageway and work area

Info

Publication number: WO2019069104A1
Application number: PCT/GB2018/052873
Authority: WO
Inventors: James Jarratt; Roger POETH
Original assignee: Highway Resource Solutions Ltd
Priority date: 2017-10-06
Filing date: 2018-10-08
Publication date: 2019-04-11
Also published as: GB201716438D0

Abstract

A monitoring system for monitoring a temporary perimeter that delineates between an open carriageway and a work area, the monitoring system comprising:multiple sensor units that can be arranged at discrete positions along the perimeter to provide a sensing array for detecting a vehicle strike on the perimeter and/or incursion into the work area, each sensor unit arranged to transmit status messages; and a classifying mechanism arranged to derive from status messages received from multiple of said sensor units a characteristic of the perimeter,e.g. it is being constructed; that construction has been completed; being deconstructed; that deconstruction is completed; that the perimeter has been reconstructed following an incursion. It can also be used to characterise an event, e.g. a car strike, on the array from the number sensors impacted and response of the sensor units, e.g. resultant time of flight, following impact.

Description

Apparatus and Method for Monitoring a Temporary Perimeter that Delineates between an Open Carriageway and Work Area

When undertaking roadworks, it is common to close off a portion of carriageway to traffic, e.g. one or more lanes, to provide a safe work area for those carrying out the works. This is commonly done using channelizers such as cones, barrels, posts, vertical panels or barricades.

Channelizers are convenient, being straightforward to carry to and from the work area and relatively cheap to replace. However they provide an ineffective physical barrier to vehicles driven unintentionally or otherwise into the work area. Various systems have been proposed to detect incursion of vehicles through roadwork perimeters. An example system, described in the applicant's earlier patent GB2486075, comprises multiple sensor units arranged to be mounted along a perimeter and which transmit a wireless signal upon detection of an impact. The system includes an alarm unit that can be positioned proximate the workers and is arranged to produce an alarm upon receiving an alert signal.

Although this system works effectively to warn workers of an incursion, improvement is possible.

According to a first aspect of the invention there is provided a monitoring system for monitoring a temporary perimeter that delineates between an open carriageway and a work area, the monitoring system comprising: multiple sensor units that can be arranged at discrete positions along the perimeter to define a sensing array for detecting damage to a temporary barrier due to a vehicle strike and/ or an incursion into the work area, each sensor unit arranged to transmit status messages; and a classifying mechanism arranged to derive from status messages received from multiple of said sensor units a characteristic of the array (and thus of the perimeter) and/or a characteristic of an event that led to the transmission of a status message.

The sensor units may be arranged to transmit a status message in response to detecting a vehicle impacting the perimeter /or a change in status of the senor unit. For example, the sensor unit may be arranged to send a status message indicative that it has entered an 'alarm mode' following detection of an impact. Examples of other conditions that may trigger the transmission of a status message include any one or more of the following: the sensor unit has been deployed; the sensor unit has been deactivated; the battery level of the sensor unit has changed and/or has past a threshold level; the location of the sensor unit has been identified or changed.

Each sensor unit may also be arranged to send status messages periodically, e.g. to provide an indication that it is functioning. Each sensor unit may additionally or alternatively be arranged to send a status message in response to an enquiry instruction, which might, for example, be received wirelessly or by activation of a switch of the sensor unit.

The classifying mechanism may be computer implemented. Expressed another way the monitoring system may comprise one or more processors and memory suitably programmed in order to implement the function of the classifying mechanism.

The classifying mechanism may hold, for an array, a record for each sensor unit in the array. Each record may include status information about the sensor unit received or derived from one or more of the status messages transmitted by the sensor units. A characteristic of the array may be determined using these records.

The classifying mechanism may also hold other information derived from the records that can be used to classify the array, such, for example one or more of: the total number of senor units in the array, time that the array has been in place and number of sensor units in a particular status, e.g. in an alarm mode. The classification mechanism further preferably holds array classification records that include associated classification attributes; and means to determine a classification by identifying from the records for an array whether the attributes for a classification are met. A characteristic of the array may be derived through any one or more of: the number, spacing and relative arrangement of the sensor units from one another. A characteristic of the array may be used to identify, or may correspond to a characteristic of the perimeter. For example, the relative arrangement of sensor units could be used to identify a tapered portion of the perimeter, e.g. through a placement pattern indicative that the sensor units extend in a diagonal direction relative to a direction of travel of traffic along a highway about which the perimeter is placed. In another example the number of sensor units may be used to identify the number of lanes closed of a multiple lane highway, e.g. whether the closure is of a single lane, two lanes, three lanes etc.. The classification mechanism may apply different rules for sensor units within one portion of the array e.g. associated with the tapered perimeter portion, compared with another portion of the array. In another example, a classification may be based on the number of sensors units that, from their transmitted status messages, have batteries with depleted batteries or that are emitting an alarm signal. The classification mechanism may also use mapping data, e.g. road mapping data, and/or geofencing, optionally with an identified characteristic of the sensing array to classify the perimeter. For example, receipt of status messages providing positional information from multiple sensor units may be used to characterise that the array extends across the whole width of the road and thus may be used to determine that the perimeter closes a whole road rather than a part of it.

Additionally or alternatively, the monitoring system may be arranged to derive a characteristic of the array and/or an event by determining a pattern from received status messages. For example the pattern may be based on one or more of the following: timings between transmission/receipt of status messages; the number of status message received; status identifiers transmitted (e.g. whether in an alarm state; has been deployed; has been deactivated) the relative position of the sensor units that transmit the signals with respect one another; and the sensor units relative position in the array.

The classifying mechanism may be arranged, for example, to characterise that the array/perimeter is in one or more of the following states: being constructed; that construction has been completed; being deconstructed; that deconstruction is completed; that the perimeter has been reconstructed following an incursion; and that the sensors units are in need of replacement/servicing which may include battery change.

For example, a classification that the construction of a perimeter is in progress may be established from a pattern comprising the receipt of multiple status messages in a specified time period indicative that sensor units have been deployed. Completion of perimeter construction may be established by a pattern comprising the receipt of multiple status messages indicative that sensor units have been deployed followed by a time period in which no deployment status messages are received that is greater than a deployment period between which two deployment status messages are expected. In such as situation the array/perimeter may be reclassified from a 'being constructed' classification to an 'in use' classification and the rules that govern what circumstances an alert may be generated may change as a result.

In addition to being able to detect an incursion, the invention allows for characteristics of a vehicle strike on the perimeter and/or incursion to be determined. Characterisation of the incursion may include identifying any one or more of: the number of sensor units that emit a status message indicative of a vehicle strike on the barrier and/or incursion, their relative placement to one another, the order in which they transmitted status messages and the relative timings of the status messages. The sensor units may also be arranged to transmit information relating to a motion characteristic of the sensor unit, for example, following an impact or otherwise indicative of a possible vehicle perimeter strike and/or incursion. Information relating to a motion characteristic may comprise, for example acceleration, direction, angle of rotation and time of flight of the sensor unit. Each senor unit impacted may be arranged to send a first status message upon detecting an impact/incursion and a second status message that includes motion characteristic information of the sensor unit at a time after the first signal.

An event classification may be made from motion characteristic information from one or more sensors units. An event may be classified by the relative speed of the impact distance travelled following impact as derived from the motion characteristic information.

The monitoring system may comprise means to generate an alert. The monitoring system may be arranged to activate the means to generate an alert.

The classification mechanism may be able to provide different alerts, associated with different level/grades e.g. depending on anticipated urgency/danger.

For example the classification mechanism may be arranged to generate a first type of alert, e.g. a low level alert in response to detection that a battery of a sensor in an array is low, a second type of alert associated with a increased degree of urgency upon a classification that batteries of multiple sensors are critically low or are no longer functioning. Selecting an action to take may be in response to the transmission of a status message using a classification of the array/perimeter that the sensor unit is associated with. For example, in response to receiving a status message indicative of a perimeter strike and/or incursion from an array that has been classified as being in a state of being constructed, the monitoring system may be arranged not to activate an alarm system, on the expectation, for example, that it is common for sensor units to be knocked over or moved by workmen in the process of construction. The status messages transmitted by the sensor units may include an identifier of the sensor unit and/or an indicator of the sensor unit type.

The monitoring system favourably comprises means to generate an alert to warn workmen within the work area of a potential incursion. The system may include an alert unit that may be arranged proximate workers in the work area, and means to activate the alert unit to provide an alert in response to detection of a potential incursion by the sensing array. The alert may be activated by the alert unit in response to receipt by the alert unit of a status message from a sensor unit of the array. This may occur without a classification process taking place. The sensor units may comprise means to transmit the status message wirelessly. To this end the sensor units may comprise wireless transceivers. The sensor units may be arranged to relay status message of other sensor units in the array.

At least some of the sensor units may comprise a motion sensor, e.g. accelerometer to detect an impact and/or ultrasonic, light or other proximity sensor to detect motion of a proximate object past the sensor unit. The sensor units may include characterising means (e.g. including a filter) arranged to characterise a motion detected by the motion sensor to differentiate motion indicative of a possible incursion from spurious motion.

The sensor unit may be arranged to be mounted to channelizers, e.g. traffic cones, on/proximate the perimeter. The sensor units may be incorporated into lanterns. The lanterns may include a lens to project light away from the lantern's light source.

The sensor unit may be arranged to transmit a status message when deployed on the perimeter. The sensor unit may comprise a switch arranged to be operated through contact with the channelizer, when the sensor unit is mounted to a channelizer, to activate the sensor unit. The monitoring system may comprise a local receiver that is relatively local to the perimeter, the local receiver arranged to receive a first status message from a sensor unit, and pass on status information to a remote receiver for use by the classifying mechanism. This allows the sensor units to comprise relative low power short range transmitters.

The local receiver may be arranged to delay transmission of status information received from one or more further status messages that are received within a threshold period immediately following receipt of the first status message. This helps to avoid the classification mechanism deriving classifications (and the actuator acting upon them) before all information from the array pertaining to change in the array has been received.

It would be possible instead for the local receiver to simply rely status messages as received and arranged for the classification mechanism to delay making a classification for a period after the last determined status message is received pertaining to a change in array classification/event.

The local receiver may simply relay the first or further status messages to the sensor. Additional or alternatively, the local receiver may send information derived from one or more of the received status messages. The invention will now be described by way of example with reference to the following figures in which:

Figure 1 is a plan view schematic of a carriageway partially closed off with a cone barrier with a sensor units mounted thereon to form a sensing array of a monitoring system; and Figure 2 is a schematic representation of the monitoring system.

With reference to Fig 1 there is shown a carriageway 1 which is partially closed off by a cone perimeter 2 to define a work area 3. The cones 4 of the cone perimeter 2 are arranged to provide a tapered portion 2A to channelize approaching traffic into open lane 1 A. (Note: this arrangement is based on left-hand-traffic.)

Mounted to each or some of the cones 4 forming the perimeter is a sensor unit 5. Together the sensor units 5 define a sensor unit array that forms part of a monitoring system able to provide warning of a vehicle strike on the perimeter and/or incursion into the work area 3, e.g. through the passage of a vehicle through the cone perimeter 2. With reference also to Fig 2, in addition to the sensor units 5, the monitoring system comprises a local receiver 6, an alarm unit 7 and a remote computing system programmed to carry out the functions of a classifying mechanism 8. The computing system includes a transceiver 9. Although shown in this embodiment as separate units, the local receiver 6 and alarm unit 7 may instead be integrated as a single unit.

Each sensor unit 5 may have a waterproof housing and a power source, e.g. a battery. In one arrangement they may be incorporated into the housing of a roadside lantern and optionally share the lantern's power source, e.g. as described in GB2486075A. Each sensor unit 5 comprises one or more sensors 5A, a processor 5B, global navigation satellite system receiver (GNS) (e.g. GPS) 5C, power supply level indicator 5D, transceiver 5E and clock 5F. The sensor unit may be incorporated into a lantern and thus be powered by a battery ((or other energy store)not shown) used to power the light source of the lantern. The sensor unit comprises a switch 5G arranged to be activated by mounting of the sensor unit on the cone to place the sensor unit in a deployed mode. Alternatively (or in addition), the switch may operable by a workman.

The one or more sensors 5 A are adapted to detect an impact on the perimeter and/or incursion into the work area 3 from the open carriageway. In an embodiment, the one or more sensors 5 A include one or more accelerometers to detect when the sensor unit 5 is impacted. The output of the accelerometers characterising the sensor unit' s 5 movement can also be used by processor 5B, together with clock 5F and GNS 5C to determine time of flight, direction of movement, number of rotations of the sensor unit when impacted. Alternatively or in addition, the one or more sensors 5A may comprise other movement and/or proximity sensors, for example an ultra sonic or an optic sensor arranged to detect the interference of a beam. Signals from the one or more sensors 5A are filtered by processor 5B in order to differentiate movement indicative of an impact likely to be a result of a potential incursion from spurious movement such as rocking or vibration as a result of heavy goods lorries passing close by at speed.

The processor 5B is arranged to receive signals from the one more sensors 5A, the navigation satellite system receiver 5C and power supply level indicator 5D and in response transmit status messages through transceiver 4E. Status messages may be sent in response to changes in the status of the sensor unit 5 for example because: the sensor unit has been powered on; powered off; the charge of the battery power is below a threshold; an impact has been detected; has been reset following detection of an impact; it has identified its location via GNS 5C etc. The status message transmitted from each sensor unit may include an indication of said status and a unique identifier of the sensor unit the initially generated the status message.

Status messages from the sensor units 5 are relayed along the perimeter 2 to the local receiver 6 by other sensor units 5 acting as intermediary nodes using a frequency- hopping protocol as is known in the art and described in GB 2486075. The sensor units 5 are arranged to send two status messages following detection by the one or more sensors 5A of an impact (or otherwise a breach in the perimeter 2). A first message is sent immediately following detection of an impact. A second (auxiliary) status message may be sent subsequently that comprises additional information characterising the impact including one or more of: time of flight of sensor unit, direction of movement of sensor unit, number of rotations of sensor unit. In order to provide a greater chance of a sensor unit 5 being able to send a auxiliary status message following an impact, the status unit's circuitry may include a capacitor that holds sufficient charge to power the sensors 5 A, processor 5B, GNS, transceiver 5C and clock 5F for a number of seconds in case the connection to the battery is severed. The local receiver 6 comprises a processor 6A, memory 6B, transceiver 6C and clock 6D. The local receiver 6 is arranged upon receiving a status message to identify if is contains an indication of a perimeter breach. If it does, the processor 6A causes the transceiver 6C to send an alert signal to the alert unit 7 sited proximate the workforce, to warn of a potential breach so that the workforce can take evasive action. It also forwards the content of the status message to the remote receiver 9.

The system may include multiple alert units which may take different forms, for example the alert unit may be a stand alone unit (e.g. place on the ground near the workmen) or be wearable.

The alert unit 7 comprises a receiver 7A, processor 7B and alert generator 7C to provide a visual and/or audible alert. The alert generator 7C may provide one or more of a visual, audible alert and to this end may include one or more of a screen, a horn and siren. Upon receipt of an alert signal by receiver 7A, the processor 7B causes the alert generator 7C to operate.

The message(s) sent to the alert unit 7 and remote receiver may be the relayed status message from the sensor unit, the status message relayed in an altered form or a separate signal derived in response to receipt of the status message.

In a high speed incursion of a motor vehicle into a cone perimeter, it is expected that multiple sensor units 5 will be impacted leading to the transmission of multiple status messages indicative of the perimeter strike and/or incursion. The local receiver 6 is arranged to enter a defer mode for a time period tl as determined by clock 6D, following receipt of a first status message, in which it stores and/or processes any further status message received within tl without forwarding them to the remote receiver 9. Each time a further signal is received within tl, the receiver is reset so that the tl period starts again. Once a period tl has expired without a further status message being received indicative of an impact, the local receiver 6 either forwards all of the further status messages to the remote receiver 9 or sends a message containing information derived from and/or pertaining to the further status messages.

In this way the system is able to react quickly to warn of a perimeter strike and/or vehicle incursion as soon as the first status message is received, whilst avoiding the classification mechanism 8 from making premature classifications (and issuing alerts based on them) before all of the data from the sensing array pertaining the impact is received.

The classifying mechanism 8, implemented by a suitable programmed combination of processor(s) and memory etc includes the functions and features of:

a classifier 8A;

an updater 8B;

for each sensor array classified by the classifying mechanism 8: a sensor unit table 8C that includes a record for each sensor unit 5 in the array holding characteristics of each sensor unit obtained or derived from status messages transmitted by the sensor units 5;

an array attribute table 8D that holds attributes of the array derived from the information held in the sensor unit table 8C and classification(s) derived from classification table 8E below;

a classification table 8E that associates classifications for an array and events affecting the array with characteristics and attributes held in the sensor unit table 8C and attribute table 8D; for each classification the classification table may also include an indication of an action to take; and

an actuator 8F.

Characteristics of the sensor units 5 held in the sensor unit table 8C may include any one or more of the following: the sensor unit's status e.g. deployed, in an alarm mode, deactivated; time entered/been in current status; location; indication of battery level; and sensor unit id. Attributes held in the array attribute table 8D may include one or more of the following: location of the array; number of sensor units in array; number of sensor units in one or more status, e.g. number of sensors in alarm mode; time since completion of construction of array; type of perimeter, e.g. derived in combination with location information and mapping data indicating if perimeter comprises a taper, or partially or completely closes a road.

Example classification, rules and associated actions that may be held in the classification table 8E are shown in table 1.

Table 1

Classification Name Rule(s) Action

Minor Strike/Incursion number of sensors in alarm Alarm type 1 (send to state = 1 one location) update graphical display.

Severe Strike/Incursion number of sensors in alarm Alarm type 2 (send to state > n+5 AND/OR two locations) sensor units changed to an Instruction to employ a alarm state within time variable speed limit of period t is greater than n + X on carriageway.

2;

Array/Perimeter being time since a sensor unit No alarm

constructed change to a deployed Instruction to employ a status < t variable speed limit of

X on carriageway.

Array/perimeter constructed time since a sensor unit Update mapping system and active change to a deployed to show perimeter in status > t (indicative that place.

the array is no longer be

constructed); Array reset during a sensor unit changed from Cancel all alarms.

maintenance to repair a an alarm state to a Instruction to employ a breached perimeter deployed state variable speed limit of

Y on carriageway.

Battery low number of sensor units Warning that

with battery level below maintenance needed. threshold > m;

A message and/or relayed status message from local receiver 6, is sent to the updater 8B and actuator 8F. Upon receipt of a message from the local receiver 6, the updater 8B updates the sensor unit table 8C and array table 8D using the information within the message. For example, if a status message from a sensor unit 5 includes location information, this information is added to relevant record in the sensor unit table 8C; the array table 8D is then updated to reflect the new information in the sensor unit table 8C.

The update to the sensor unit table 8C prompts the classifier 8A to derive a classification of the array from classification table 8E using the information in sensor unit table 8C and array table 8D, and updates the array table 8D to update the determined classification of the array (if changed).

The actuator 8F also prompted by receipt of message from the local receiver 6 identifies an action to perform from the classification of the array, which might for example include one or more of causing: an alert message to be sent via transceiver 9 to a first destination 10 (e.g. send a text message or ring a first phone); sending an alert to a first and second destination (e.g. send message and/or ring two different phones); changing a variable speed limit proximate the work area, e.g. by sending signal to change speed limited displayed to motorist in area proximate work area; updating a graphical display showing status of perimeter; sending a broadcast for receipt by navigational system of vehicles (including autonomous vehicles approaching the road works); and sending a signal (e.g. via transceiver 9) to deactivate the alert unit 7.

In an example situation in which a vehicle impacts multiple sensor units, each of the sensor units 5 transmits a first signal indicative that it is registered an impact. Upon receipt of the first of these signals, the local receiver transmits an alarm signal to alarm unit 7 that in response generates an alarm, e.g. activates one or more visual or audible alerts. The message received by transceiver 9 is used by updater 8A to update the senor unit table for the array to indicate that the sensor unit is in an alarm mode, the array table is also updated, and the classifier then derives a new classification of the array. The classification from Table 1 based on one sensor unit in an array being in an alarm state is a Minor Incursion. The actuator, also prompted by receipt of the message carries out the action associated with the classification, which may be for example, sending a text message or calling a first mobile number and updating a graphical display that illustrates the perimeter on a controller's screen, updating traffic management system or broadcasting message to nearby autonomous/connected vehicles.

Following receipt of the first status message, the local receiver 6 enters a store mode, such that the status messages received from the other sensor units also impacted are stored. Following expiry of time tl, the local receiver 6 transmits a message(s) based on the further status message received whilst in the store mode. The messages(s) received by transceiver 9 are send to the updater 8B and actuator 8F. The array is reclassified and the actuator takes the action appropriate to the new classification, which may for example being Severe Incursion.

Following an impact each sensor unit impacted sends a further status message following the initial status message that includes motion characteristic information. The motion characteristic information is forwarded by the local receiver 6 via transceiver 9 to the classifying mechanism 8. The motion characteristic information for each sensor unit is included in added to the sensor unit table. The classifier 8 A includes an event classifier 11, arranged in response to the addition of motion characteristic information (or other information pertaining to an event where the system is so arranged) to derive a classification of the event using event classifiers held in the classification table 8E. The event may be classified for example based on the number of sensor units affected and the magnitude of the impact. For example an impact in which each of the sensor units moved, following impact in a way characteristic of being impacted at relatively slow speed e.g. 20mph maybe characterised as a low speed impact, whereas motion information characteristic of having been impacted at high speed e.g. 70mph, or where no further motion characteristic information was received by multiple sensor unit indicative that the impact was sufficient to render the sensor unit inoperable, may be characterised as a high speed impact. Different event classifications may have different actions associated therewith. The classification of the event is used by the actuator 8F to carry out an action based on that associated with the relevant classification in the classification table 8E. Taking the above exampled classifications of low speed and high speed impact, a low speed impact may have an action to issued a medium priority alert, e.g. via a text message to an site manager's phone, that attendance is required at the impact site (for this the GNS info from the affected sensor units may be used) to repair the perimeter. The action for a high speed impact may be to issue a high priority alert, that urgent attendance is required at the impact site, it being assumed that there is a high chance that some of the cones and sensor units will have been struck hard enough to have moved into the open carriageway.

Following attendance by maintenance workers to the scene of the incursion, the righting of the sensor units will prompt the transmission of a status messages indicating the sensor unit is in a deployed state. In response to a change of classification to a reset status, the actuator 8F may be cause to cancel the alarms.

In a variant system, a local receiver may be omitted and the sensor units arranged to communicate directly with transceiver 9 and/or alarm unit 7. In a further variant the sensor units may omit the GNS receiver. Instead the approximate position of the sensor units, and thus the perimeter may be deduced by virtue of the position of the local receiver (equipped with a GNS receiver and which provides its position to the remote computing system) that passes on status information from the sensor units. Further location information of each of the sensor units can be deduced from time-of-flight ranging from each sensor unit to the local receiver, and the order of deployment of the sensor units when used in conjunction with knowledge of conventions used in channelizer placement, number (16 or 17 for a single lane taper (motorway in United Kingdom) and separation (3 meters in the United Kingdom) for various types of perimeter.

The classifying mechanism may be arranged to classify an event only or the array only. The one or more sensors 5 A may in addition or alternatively include other sensors adapted to detect an incursion into the perimeter, e.g. proximity sensors for example optic or ultrasonic. A signal received by the processor 5B from these sensors indicative of an incursion may cause the sensor unit 5 to adopt an alarm mode status and for the relevant status message associated with this to be transmitted for classification by classifying mechanism 8.

The functions of an alert unit may instead be incorporated into an electronic device carried by the workmen, such as a smartphone or watch and use the electronic device's own speaker and/or screen to provide the alert. In such case the alert may also include activation of the electronic device's vibrate function.

Claims

1. A monitoring system for monitoring a temporary perimeter that delineates between an open carriageway and a work area, the monitoring system comprising: multiple sensor units that can be arranged at discrete positions along the perimeter to provide a sensing array for detecting a vehicle strike on the perimeter and/or incursion into the work area, each sensor unit arranged to transmit status messages; and a classifying mechanism arranged to derive from status messages received from multiple of said sensor units a characteristic of the array, and/or a characteristic of an event that led to the transmission of a status message.

A monitoring system according to claim 1 wherein the classifying mechanism holds a record comprising status information for each sensor unit in the sensing array that is populated from received status messages, and in which a characteristic of the perimeter and/or array, and/or a characteristic of an event that led to the transmission of an status message is derived from or using said records.

A monitoring system according to claim 1 or 2 wherein the classifying mechanism is arranged to derive a characteristic by determining a pattern from the received status messages.

4. A monitoring system according to claim 1, 2 or 3 wherein the classifying mechanism is arranged to derive a characteristic from a time period between status messages transmitted from at least two of the multiple sensor units.

5. A monitoring system according to any previous claim wherein the classifying mechanism is arranged to derive a characteristic of the array from the relative positions of the sensor units along the perimeter.

6. A monitoring system according to claim 5 wherein the classifying mechanism is arranged to derive a characteristic from a pattern derived from the relative position of sensor units that have transmitted a status message.

7. A monitoring system according to any previous claim wherein each of the sensor units are arranged to transmit a status message comprising motion characteristic information.

8. A monitoring system according to any previous claim comprising alert generating means arranged to generate an alert in response to a status message transmitted from the array indicative of a perimeter strike and/or incursion.

9. A monitoring system according to any previous claim comprising alert generating means arranged to generate an alert in response to a characteristic of the perimeter and/or array that is associated with a need for remedial action.

10. A monitoring system according to any previous claim comprising a local receiver arranged to be relatively local to the perimeter, the local receiver arranged to receive a status messages from the sensing array, and pass on status information provided through the status messages to a remote receiver for use by the classifying mechanism.

11. A monitoring system according to claim 10 wherein the local receiver is arranged to delay transmission of status information included in and/or derived from further status message received within a threshold period immediately following receiving a first status message.

12. A method of monitoring a temporary perimeter that delineates between an open carriageway and a work area, the monitoring system comprising: multiple sensor units that can be arranged at discrete positions along the perimeter to provide a sensing array for detecting a vehicle strike on the perimeter and/or incursion into the work area, each sensor unit arranged to transmit status messages; and the method comprising deriving from status messages received from multiple of said sensor units a characteristic of the array, and/or a characteristic of an event that led to the transmission of a status message.